Many three-axis stabilized spacecraft utilize solar wings to generate electrical energy. In order to maximize the energy output, the solar wings must be properly oriented towards the Sun. Spacecraft also often use radiators to eliminate waste heat. These radiators must not be oriented towards the Sun in order to properly function. Spacecraft may also have mission objectives that require a specific attitude relative to the Sun and other celestial bodies such as the Earth.
Time and energy is expended by a spacecraft when modifying its attitude. Minimizing time avoids thermal problems and reduces battery drainage. Minimizing energy expenditure conserves the limited amount of fuel normally carried by a spacecraft.
Current methods acquire a solar-pointing attitude by slewing the spacecraft one axis at a time. A typical solar orientation may require three phases: pitch search, keyhole maneuver and yaw search. Consider, for example, the following typical sequence of actions and associated execution times.
The spacecraft first performs a pitch scan to search for the Sun. A 360 degree pitch scan at a typical search rate of 0.75 degrees per second takes 8 minutes. During the pitch search phase, the solar wings are slewed to face the roll axis. If the pitch search is unsuccessful, the Sun is assumed to be in the sensor blind spot or keyhole. In this case, a one time 75 degree yaw slew at a typical search rate of 0.25 degrees per second takes 5 minutes. Adding 8 minutes for a second pitch search yields 13 minutes for the keyhole maneuver. Once the spacecraft locks onto the Sun in pitch, the spacecraft performs a yaw scan while maintaining Sun-lock in pitch. A 45 degree yaw slew at a typical search rate of 0.25 degrees per second takes 3 minutes. The total maximum time for the above procedures is 24 minutes.
Modifications to the above sequence have been proposed, but none significantly reduce the worst-case acquisition time.
An additional problem with existing solar acquisition methods is that the large search rate about the body pitch axis generally requires large momentum wheel storage requirements.
A further problem occurs in Sun-nadir steered spacecraft, where Earth-lock can be lost during the solar acquisition sequence.